This invention relates to protective systems for electric motors. More specifically, it relates to overload, overtemperature, phase unbalance, overcurrent, and ground fault protection for electric motors.
Unnecessary motor shut-downs from premature relay trips can rob a critical process, industry, or utility of valuable operating time. A relay that does not trip soon enough can result in a heat damaged or burned-out motor which also results in lost operating time. Critical operations that require high reliability drives need motors that have precise protection.
A bimetal type relay provides overload protection against motor overheating under steady state conditions. Bimetal relays, however, cannot accurately reproduce the temperature of motor elements under transient and cyclic conditions of short time durations. Therefore, bimetal relays do not provide good protection for repeated starts or hot starts and may trip prematurely on cyclic loads.
Most large motors have resistance temperature detectors placed in the stator slots and near bearings. Resistance temperature detectors (RTD's) are essentially resistance elements of copper, platinum or nickel. Measuring the resistance of the RTD's installed in the motor gives an indication of motor winding and bearing temperatures. The use of a thermal analog of a motor having motor current inputs and winding temperature inputs from RTD's closely approximates the heating characteristics of most motors during both starting and steady state operations. The IEEE paper Thermal Tracking--A Rational Approach to Motor Protection T74029-5 discusses a thermal analog using RTD's and motor current inputs that precisely tracks motor temperature changes precisely as they occur; rapidly during stall, more slowly during gradual load increases, and takes accurate account of temperatures during cooling. The IEEE paper is hereby incorporated by reference.
A relay using an accurate thermal analog of a motor, while a vast improvement over the bimetal type relay, has some disadvantages. One disadvantage is that it is not readily adjustable in the field. Many times the calculated motor data, given initially by the motor designer, does not agree with the actual motor data obtained in tests in the field. These differences necessitate adjustment of the relay which requires returning the relay unit to the factory for adjustment or the use of expensive field equipment for adjustment.
Another disadvantage to the thermal analog relays is that should an RTD open, the high resistance of the open RTD input would then be mistakenly interpreted as indicating a very high temperature which in turn would cause a premature trip of the motor.
An object of the present invention is to provide a motor protection device that has field adjustable set points, for the various functions monitored, that do not require special instrumentation or tools to enter.
Another object of the present invention is to provide a motor protection device that can monitor more than one winding RTD and more than one bearing RTD.
Another object of the present invention is to provide a motor protection device that disregards open RTD's.